Halve the onion and discard the central-most part. Cut halves into thin slices.

Heat oil in a large skillet or wok over high heat. Add onion; cook and stir until it starts to brown, about 30 seconds. Reduce heat to medium-low; add water, soy sauce, brown sugar, mirin, and sake and simmer until flavors combine, about 3 minutes.

Stir beef into the skillet. Cook, covered, until beef is cooked through, 3 to 5 minutes. Divide between serving bowls and garnish with sesame seeds, green onions, ginger, and seaweed strips.